This invention is related to novel flocculating agents for aqueous dispersions of mineral wastes, their preparation and method of use, particularly for flocculating the bauxite residue from the production of alumina (red mud).
The production of alumina from bauxite is carried out by use of the well-known Bayer process. Essentially, the Bayer process is an alkaline extractive hydrometallurgical process which involves dissolution of the aluminum oxyhydroxides, gibbsite (Al(OH).sub.3) and boehmite and/or diaspore (Al.sub.2 O.sub.3. H.sub.2 O) present in bauxite, with caustic soda at elevated temperatures and pressures. The process is based on the variation in the solubility of gibbsite, boehmite and diaspore with increasing caustic soda concentration.
In practice, bauxite is slurried in caustic soda solution and digested at temperatures ranging from 140.degree. to 250.degree. C. This results in the formation of a slurry consisting of a saturated sodium aluminate (Na Al (OH).sub.4) liquid phase and a caustic insoluble bauxite residue which is commonly called red mud or ferrosilt. Subsequent processing involves separation of the solid phase from the saturated sodium aluminate liquor by use of decantation or other techniques. The liquid phase is further clarified by filtration to remove the last traces of red mud and is finally transported to the precipitation or decomposition operation in which previously precipitated gibbsite seeds (crystals) are added to induce precipitation. The gibbsite generated is washed and calcined at temperatures ranging from 1000.degree. to 1200.degree. C. to drive off free moisture and water of crystallization. The calcined alumina is cooled, stored and transported to smelters.
The red mud produced is washed in a counter current decantation (CCD) system to recover valuable solutes associated with it. The red mud is finally discharged from the battery limits of the alumina plant into impoundment areas which may be natural depressions, mined out bauxite pits or artificial lakes, in the case of disposal on land.
The red mud residues produced in the alumina recovery process comprise very fine particles which are very difficult to remove, yet for the sake of efficiency of the process these red muds, which usually constitute from about 5% to about 30% of the ore, must be quickly and cleanly removed from the solubilized alumina liquor.
As disclosed in U.S. Pat. No. 3,390,959, it is known in the industry to speed up the separation of red muds from alumina liquors by adding various flocculating agents such as starch or starch derivatives, acrylic-acid-acrylate homopolymers, acrylamide derivatives, imidazoles and various other agents. However, these agents are not fully acceptable for one or more reasons: e.g. cost, effectiveness, causation of problems in further processing, and the like.
This invention provides a novel agent for flocculation of red mud in alumina liquors and similar highly dispersed aqueous systems which gives significantly improved separation of the dispersed particulates and thus enables more efficient processing to be obtained.